This invention relates to magnetically enhanced sputtering devices and, in particular, to improved means for implementing the magnetic enhancement of such devices.
Generally, crossed magnetic and electric fields are established in such devices. The electric field extends between an anode (which may be the chamber wall) and a target, which is typically at cathode potential and in circuit with the anode, whereby electrons are removed from the target. The removed electrons ionize gas particles to thereby produce a plasma. The ions are accelerated to the target to dislodge atoms of the target material. The dislodged target material then typically deposits as a coating film on an object to be coated. In order to improve the sputtering rate at low gas pressures, the crossed magnetic field is provided to lengthen the path travelled by the removed electrons and thus enhance the ionizing efficiency of the electrons. In order to further improve the ionizing efficiency of the electrons, a closed loop plasma path is preferably established so that a Hall effect current circulates around the path.
The ionizing electrons tend to concentrate in the regions where the magnetic lines of force are parallel to the target surface. In prior art devices which employ a closed plasma loop, the region over which the magnetic lines of force are parallel to the target surface tends to be rather small thus promoting non-uniformity of target erosion and inhibiting the realization of higher sputter rates. This is illustrated in FIGS. 3A and 3B where the conventional erosion pattern tends to be V-shaped. Further, target utilization tends to be minimal in that a relatively small portion of the target is sputtered before the erosion breaks through the bottom destroying it.
The exact mechanisms for magnetically enhanced plasma generation and retention against cathode surfaces are not fully understood. However it has been observed that the sputter erosion of target areas relates rather positively with the intensity--thickness product of the plasma above them. The plasma will not under all conditions cause erosion beneath itself, but there will certainly be no erosion where there has been no plasma. Progressing from gentle glow discharges toward intense plasmas, the plasma pattern widens but the intensity thereof tends to centralize. The resulting erosion track tends to be V-shaped as indicated in FIGS. 3A and 3B. Using a magnetic field source disposed beneath the target, this can be flattened somewhat in the bottom by shaping the magnetic field lines over the track to thereby inhibit somewhat the centralizing tendency in the plasma. When it is realized the track may be a loop on a planar target surface and the magnetic field must reverse polarity at the center and outer edges, there is little room to achieve such magnetic reversals at significant gauss levels and to also provide the desired degrees of field shaping. Thus, when the magnetic field source is disposed beneath the target, it is difficult to fully flatten the lines of force to encourage plasma spread.
It is thought the plasma exists primarily where effective trapping of ionizing electrons has been provided or if they have not been trapped, they have at least been provided in significant density. When the plasma forms, its nearly equal numbers of electron and positive ions provide a low impedance region that minimizes the voltage gradient through it. This decreased gradient reduces the electrostatic forces that would scatter the electrons, making for greater plasma density. The increased density further lowers the electrostatic scattering. Thus the plasma formation is a positive feedback phenomenon. Hence, there is little flexibility available in prior art approaches, such as those discussed with respect to FIGS. 1 and 3 of aforementioned application Ser. No. 935,358, to lessen the tendency for the undesired V-shaped erosion of the target.